This invention relates, in general, to current generator stabilisation circuits and is particularly, but not exclusively, applicable to laser driver circuits operating in voltage environments having restricted voltage headroom.
In the control of laser diodes devices and the like, it is imperative that accurate current control be exercised. Indeed, to establish accurate laser operation, it is necessary to ensure that currents are stable and unaffected by integrated circuit (IC) process parameters. More specifically, it will be appreciated that the xe2x80x9cextinction ratioxe2x80x9d of the lasing device must be kept constant. As will be understood, the extinction ratio is the current ratio between a first current yielding a light level attributed to a logical zero value and a second current yielding a light level attributed to a logical one value. Moreover, it is an ac modulation current to the laser diode that provides a logic level control mechanism through which transitions between logical states are controlled, the ac modulation current defined between the first current and the second current. Typical extinction ratios are between about 10:1 or 20:1.
For completeness, it will be appreciated that lasing operation will not commence before a minimum current threshold (Ithres) has been surpassed and that logical zero is defined by a current greater than Ithres. Furthermore, laser diode operation (having regard to the ac modulation current range) varies as a function of temperature.
Commercial laser diode systems operate in the giga Hertz range (GHz) and employ bi-polar transistor technology to allow use of base current leakage. In a move to control power dissipation, however, operating platforms are moving towards lowering supply voltages initially from five volts (5 V) to 3.3 V and below. With the supply providing current to be drawn by the laser diode, a conventional current mirror circuit compensates for current error due to base current by employing an emitter-follower structure; this stabilises laser current supply. Current error is particularly prevalent in circuits having high mirror ratio, say 12:1, current mirrors where the current base (Ibase) problem is exacerbated.
UK Patent application 2 335 556 describes an emitter-follower arrangement in a BiMOS switched current source for a digital to analog converter (DAC) or phase lock loop (PLL) charge pump.
Unfortunately, to date, transistor current mirror compensation techniques (employing multiple transistors) require voltage headroom that may exceed the inherent capabilities of the supply voltage Vcc, with this particularly true in the context of silicon-germanium (SiGe) transistors where there is a trend to increased base-emitter junctions voltages, Vbe. While the current base problem is most noticeable in pnp structures, npn structures are nevertheless susceptible.
According to a preferred embodiment of the present invention there is provided a current mirror circuit of a laser driver circuit, the current mirror circuit comprising: first and second mirror transistors each having a base electrode and an emitter electrode; first and second emitter resistors coupled, respectively, to the emitter electrodes of the first and second mirror transistors, and characterised by: a resistive element coupled between said base electrodes such that, in use, base leakage current to the first transistor from a current supply causes a voltage drop across the resistive element, wherein the voltage drop acts to boost a voltage through the emitter resistor of the second mirror transistor, thereby to boost, in use, output current provided by the second mirror transistor to the laser driver circuit.
In a second aspect of the present invention there is provided a method of providing drive current in a laser diode circuit, the method comprising: providing an input current to an input stage of a current mirror, the input current provided to a collector electrode of a first transistor; and characterised by: coupling together base electrodes of the first transistor and a second transistor of an output stage of the current mirror, the coupling occurring through a resistive element arranged to cause generation of a voltage drop across the resistive element; and modulating an output current from the output stage of the current mirror, the output current boosted by the voltage drop across the resistive element to provide the drive current.
The output current provides current to a current modulator arranged to control an extinction ratio associated with operation of a laser diode.
In a further aspect of the present invention there is provided a resistive beta compensation current mirror arranged to boost output current supplied to a laser diode having an operational power supply that is less than about 5 volts.
Another aspect of the present invention provides use of a resistive beta compensation current mirror to boost output current supplied to a laser diode having an operational power supply that is less than about 5 volts, the laser diode preferably supported by a silicon-germanium bipolar transistor semiconductor technology.
In still yet another aspect of the present invention there is provided a current mirror circuit connectable, in use, to a laser driver circuit, the current mirror circuit comprising: first and second mirror transistors each having a base electrode and an emitter electrode; first and second emitter resistors coupled, respectively, to the emitter electrodes of the first and second mirror transistors, and characterised by: a resistive element coupled between said base electrodes such that, in use, base leakage current to the first transistor from a current supply causes a voltage drop across the resistive element, wherein the voltage drop acts to boost a voltage through the emitter resistor of the second mirror transistor, thereby to boost, in use, output current provided by the second mirror transistor to the laser diode circuit.
The present invention applies resistive beta compensation mirrors to control accurately, for example, laser diodes operating from relatively low supply voltages. Advantageously, the present invention therefore provides an improvement in stabilisation of a current generator output employing a current mirror in a low supply voltage environment, especially in a laser driver circuit environment.